Sensor arrays are used, for example, in video cameras, and generally include a two dimensional array of pixels that is fabricated on a substrate. Each pixel includes a sensing element (e.g., a photodiode) that is capable of converting a portion of an optical (or other radiant source) image into an electronic (e.g., voltage) signal, and access circuitry that selectively couples the sensing element to control circuits dispose on a periphery of the pixel array by way of address and signal lines. The access circuitry typically includes metal address and signal lines that are supported in insulation material deposited over the upper surface of a semiconductor substrate, and positioned along the peripheral edges of the pixels to allow light to pass between the metal lines to the sensing elements through the insulation material. Most image sensors typically contain a large number (e.g., millions) of pixels which transform photons coming from a photographed scene or other optical image source into a large number of corresponding voltage signals, which are stored on a memory device and then read from the memory device and used to regenerate the optical image on, for example, a liquid crystal display (LCD) device.
There are two main types of image sensors: Charge-Coupled Device (“CCD”) sensors and Complementary Metal Oxide Semiconductor (“CMOS”) sensors. Early CMOS sensors suffered from poor light sensitivity and high noise levels that limited their usefulness, and the majority of early image sensors were of the CCD type. Recent advances in CMOS technology have led to the development of high performance CMOS sensors that are quickly replacing CCDs in a host of other applications, particularly in those where speed, power consumption, size, and on-chip functionality are important factors.
Endoscopy is a minimally invasive diagnostic medical procedure that is used to assess the interior surfaces of an organ by inserting an endoscope into the body. There are several types of endoscopes, but the present invention is particularly directed to endoscopes that have a flexible tube having a very small image sensor at its tip, which is connected to a host instrument device by a long cable (e.g., 14 feet) made up of a set of electrical wires that extend between the device and the sensor through the tube. Such endoscopes utilize small image sensors that are mounted in the tip, and are connected by wires to a host measurement device that stores image information generated by the image sensor. The endoscope often includes small working tools along with the image sensor at the tip. Such endoscopes are required to enter smaller and smaller organs such as tiny blood vessels, and thus the image sensors utilized in endoscopes are becoming smaller and smaller. When the tip is inserted inside a patient's body, the image sensor becomes the doctor's “eyes” within the patient's body. The challenge in this case is for the image sensor to be small enough to fit together with the rest of working tools inside very narrow cable. In addition, there is a need to reduce the number of contact pads on the image sensor in order to minimize the number of wires within the tube.
What is needed is a low cost, very small area CMOS image sensor with that overcomes the problems associated with conventional very small area image sensors.